Slim's are a little more tapered than NS's, though the RRLS are a little fuller in the top. Straights need to be sized down about 2 from true waist, and then end up fitting a little slimmer, albeit slightly wider at the hem than 501's.

Do you guys were your belt over or under the leather patch on your RRL? I have always just put my belt over the patch on jeans, but the leather on the RRLs is pretty think and they seem to fit just a bit better with the belt through the patch.